Steerable drill string

ABSTRACT

A rotatable steerable drill string in which guidance module controls the direction of the drilling. A magnetorheological fluid in the module supplies pressure to pistons that apply forces to the wall of the bore and thereby alter the direction of the drilling. The pressure applied by the magnetorheological fluid is regulated by valves that apply a magnetic field to the fluid so as to increase or decrease its fluid shear strength thereby controlling the actuation of the pistons and the direction of the drilling.

FIELD OF THE INVENTION

[0001] The current invention is directed to an apparatus and method forsteering a device through a passage, such as the steering of a drillstring during the course of drilling a well.

BACKGROUND OF THE INVENTION

[0002] In underground drilling, such as gas, oil or geothermal drilling,a bore is drilled through a formation deep in the earth. Such bores areformed by connecting a drill bit to sections of long pipe, referred toas a “drill pipe,” so as to form an assembly commonly referred to as a“drill string” that extends from the surface to the bottom of the bore.The drill bit is rotated so that it advances into the earth, therebyforming the bore. In rotary drilling, the drill bit is rotated byrotating the drill string at the surface. In any event, in order tolubricate the drill bit and flush cuttings from its path, pistonoperated pumps on the surface pump a high pressure fluid, referred to as“drilling mud,” through an internal passage in the drill string and outthrough the drill bit. The drilling mud then flows to the surfacethrough the annular passage formed between the drill string and thesurface of the bore.

[0003] The distal end of a drill string, which includes the drill bit,is referred to as the “bottom hole assembly.” In “measurement whiledrilling” (MWD) applications, sensors (such as those sensing azimuth,inclination, and tool face) are incorporated in the bottom hole assemblyto provide information concerning the direction of the drilling. In asteerable drill string, this information can be used to control thedirection in which the drill bit advances.

[0004] Various approaches have been suggested for controlling thedirection of the drill string as it forms the bore. The direction inwhich a rotating drill string is headed is dependent on the type of bit,speed of rotation, weight applied to the drill bit, configuration of thebottom hole assembly, and other factors. By varying one or several ofthese parameters a driller can steer a well to a target. With the widespread acceptance of steerable systems in the 1980's a much higher levelof control on the direction of the drill string was established. In thesteerable system configuration a drilling motor with a bent flexcoupling housing provided a natural bend angle to the drill string. Thedrill bit was rotated by the drilling motor but the drill string was notrotated. As long as the drill string was not rotated, the drill wouldtend to follow this natural bend angle. The exact hole direction wasdetermined by a curvature calculation involving the bend angle andvarious touch points between the drill string and the hole. In thismanner the bend angle could be oriented to any position and thecurvature would be developed. If a straight hole was required both thedrill string and the motor were operated which resulted in a straightbut oversize hole.

[0005] There were several disadvantages to such non-rotating steerabledrill strings. During those periods when the drill string is notrotating, the static coefficient of friction between the drill stringand the borehole wall prevented steady application of weight to thedrill bit. This resulted in a stick slip situation. In addition, theadditional force required to push the non-rotating drill string forwardcaused reduced weight on the bit and drill string buckling problems.Also, the hole cleaned when the drill string is not rotating is not asgood as that provided by a rotating drill string. And drilled holestended to be tortuous.

[0006] Rotary steerable systems, where the drill bit can drill acontrolled curved hole as the drill string is rotated, can overcome thedisadvantages of conventional steerable systems since the drill stringwill slide easily through the hole and cuttings removal is facilitated.

[0007] Therefore it would also be desirable to provide a method andapparatus that permitted controlling the direction of a rotatable drillstring.

SUMMARY OF THE INVENTION

[0008] It is an object of the current invention to provide a method andapparatus that permitted controlling the direction of a rotatable drillstring. This and other objects is accomplished in a guidance apparatusfor steering a rotatable drill string, comprising A guidance apparatusfor steering a rotatable drill string through a bore hole, comprising(i) a housing for incorporation into the drill string, (ii) a movablemember mounted in the housing so as to be capable of extending andretracting in the radial direction, the movable member having a distalend projecting from the housing adapted to engage the walls of the borehole, (iii) a supply of a magnetorheological fluid, (iv) means forpressurizing the magnetorheological fluid, (v) means for supply thepressurized rheological fluid to the movable member, the pressure of therheological fluid generating a force urging the movable member to extendradially outward, the magnitude of the force being proportional to thepressure of the Theological fluid supplied to the movable member, and(vi) a valve for regulating the pressure of the magnetorheological fluidsupplied to the movable member so as to alter the force urging themovable member radially outward, the valve comprising means forsubjecting the magnetorheological fluid to a magnetic field so as tochange the shear strength thereof. In a preferred embodiment of theinvention, the fluid is a magnetorheological fluid and the valveincorporates an electromagnetic for generating a magnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic diagram of a drilling operation employing asteerable rotating drill string according to the current invention.

[0010]FIG. 2 is a cross-section taken through line II-II shown in FIG. 1showing the steering of the drill string using a guidance moduleaccording to the current invention.

[0011]FIG. 3 is a transverse cross-section through the guidance moduleshown in FIG. 1.

[0012]FIG. 4 is a longitudinal cross-section taken through line IV-IVshown in FIG. 3.

[0013]FIG. 5 is a view of one of the covers of the guidance moduleviewed from line V-V shown in FIG. 3.

[0014]FIG. 6 is a transverse cross-section through the guidance moduletaken through line VI-VI shown in FIG. 3.

[0015]FIG. 6a is a cross-section taken through circular line VIa-VIashown in FIG. 6 showing the arrangement of the valve and manifoldsection of the guidance module if it were split axially and laid flat.

[0016]FIG. 7 is a transverse cross-section through the guidance moduletaken through line VII-VII shown in FIG. 3.

[0017]FIG. 8 is a transverse cross-section through the guidance moduletaken through line VIII-VIII shown in FIG. 3.

[0018]FIG. 9 is a transverse cross-section through the guidance moduletaken through line IX-IX shown in FIG. 3 (note that FIG. 9 is viewed inthe opposite direction from the cross-sections shown in FIGS. 6-8).

[0019]FIG. 10 is an exploded isometric view, partially in cross-section,of a portion of the guidance module shown in FIG. 3.

[0020]FIG. 11 is a longitudinal cross-section through one of the valvesshown in FIG. 3.

[0021]FIG. 12 is a transverse cross-section through a valve taken alongline XII-XII shown in FIG. 11.

[0022]FIG. 13 is a schematic diagram of the guidance module controlsystem.

[0023]FIG. 14 is a longitudinal cross-section through an alternateembodiment of one of the valves shown in FIG. 3.

[0024]FIG. 15 is a transverse cross-section through a valve taken alongline XV-XV shown in FIG. 14.

[0025]FIG. 16 shows a portion of the drill string shown in FIG. 1 in thevicinity of the guidance module.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] A drilling operation according to the current invention is shownin FIG. 1. A drill rig 1 rotates a drill string 6 that, as isconventional, is comprised of a number of interconnected sections. Adrill bit 8, which preferably has side cutting ability as well asstraight ahead cutting ability, at the extreme distal end of the drillstring 6 advances into an earthen formation 2 so as to form a bore 4.Pumps 3 direct drilling mud 5 through the drill string 6 to the drillbit 8. The drilling mud 5 then returns to the surface through theannular passage 130 between the drill string 6 and the bore 4.

[0027] As shown in FIGS. 1 and 2, a guidance module 10 is incorporatedinto the drill string 6 proximate the drill bit 8 and serves to directthe direction of the drilling. As shown in FIGS. 3 and 4, in thepreferred embodiment, the guidance module 10 has three banks of pistons12 slidably mounted therein spaced at 120° intervals, with each bank ofpistons comprising three pistons 12 arranged in an axially extendingrow. However, a lesser number of piston banks (including only one pistonbank) or a greater number of piston banks (such as four piston banks)could also be utilized. In addition, a lesser number of pistons could beutilized in each of the banks (including only one piston per bank), aswell as a greater number. Moreover, the piston banks need not be equallyspaced around the circumference of the drill string.

[0028] Preferably, the pistons 12 are selectively extended and retractedduring each rotation of the drill string so as to guide the direction ofthe drill bit 8. As shown in FIG. 2, the first bank of pistons 12′,which are at the 90° location on the circumference of the bore 4, areextended, whereas the second and third banks of pistons 12″ and 12′″,which are at the 210° and 330° locations, respectively, are retracted.As a result, the first bank of pistons 12′ exert a force F against thewall of the bore 4 that pushes the drill bit 8 in the opposite direction(i.e., 180° away in the 270° direction). This force changes thedirection of the drilling. As shown in FIG. 1, the drill bit isadvancing along a curved path toward the 90° direction. However,operation of the pistons 12 as shown in FIG. 2 will cause the drill bitto change its path toward the 270° direction.

[0029] Since the drill string 6 rotates at a relatively high speed, thepistons 12 must be extended and retracted in a precise sequence as thedrill string rotates in order to allow the pistons to continue to pushthe drill string in the desired direction (e.g., in the 270° direction).For example, as shown in FIG. 2, after the pistons 12′ in the firstpiston bank reach the 90° location, at which time they are fullyextended, they must begin retracting so that they are fully retracted bythe time the drill string rotates 120° so as to bring them to the 330°location. The pistons 12″ in the second piston bank, however, must beginextending during this same time period so that they are fully extendedwhen they reach the 90° location. The pistons 12′″ in the third pistonbank remain retracted as the drill string 6 rotates from the 330°location to the 210° location but then begin extending so that they tooare fully extended when they reach the 90° location. Since the drillstring 6 may rotate at rotational speeds as high as 25° RPM, thesequencing of the pistons 12 must be controlled very rapidly andprecisely. According to the current invention, the actuation of thepistons 12 is controlled by magnetorheological valves, as discussedfurther below.

[0030] Alternatively, the guidance module 10 could be located moreremotely from the drill bit so that operation of the pistons 12 deflectsthe drill pipe and adds curvature to the bottom hole assembly, therebytilting the drill bit. When using this approach, which is sometimesreferred to as a “three point system,” the drill bit need not have sidecutting ability.

[0031] A preferred embodiment of the guidance module 10 is shown indetail in FIGS. 3-13. As shown best in FIGS. 3 and 4, the guidancemodule 10 comprises a housing 14, which forms a section of drill pipefor the drill string, around which the three banks of pistons 12 arecircumferentially spaced. Each bank of pistons 12 is located within oneof three recesses 31 formed in the housing 14. Each piston 12 has aarcuate distal end for contacting the surface of the bore 4. However, insome applications, especially larger diameter drill strings, it may bedesirable to couple the distal ends of the pistons together with acontact plate that bears against the walls of the bore 4 so that all ofthe pistons 12 in one bank are ganged together. Each piston 12 has ahollow center that allows it to slide on a cylindrical post 18projecting radially outward from the center of a piston cylinder 19formed in the bottom of its recess 31.

[0032] The radially outward movement of the pistons 12 in each pistonbank is restrained by a cover 16 that is secured within the recess 31 byscrews 32, shown in FIG. 5. Holes 27 in the cover 16 allows the distalends of the pistons to project radially outward beyond the cover. Inaddition, in the preferred embodiment, four helical compression springs20 are located in radially extending blind holes 21 spaced around thecircumference of each piston 12. The springs 20 press against the cover16 so as to bias the pistons 12 radially inward. Depending on themagnitude of the force urging the pistons 12 radially outward, which isapplied by a magnetorheological fluid as discussed below, the pistonsmay be either fully extended, fully retracted, or at an intermediateposition. Alternatively, the springs 20 could be dispensed with and themagnetorheological fluid relied upon exclusively to extend and retractthe pistons 12.

[0033] Three valve manifold recesses 33 are also spaced at 120°intervals around the housing 14 so as to be axially aligned with therecesses 31 for the piston banks but located axially downstream fromthem. A cover 17, which is secured to the housing 14 by screws 32,encloses each of the valve manifold recesses 33. Each cover 17 forms achamber 29 between it and the inner surface of its recess 33. Asdiscussed below, each of the chambers 31 encloses valves and manifoldsfor one of the piston banks.

[0034] According to the current invention, the guidance module 10contains a supply of a magnetorheological fluid. Magnetorheologicalfluids are typically comprised of non-colloidal suspensions offerromagnetic or paramagnetic particles, typically greater than 0.1micrometers in diameter. The particles are suspended in a carrier fluid,such as mineral oil, water or silicone oil. Under normal conditions,magnetorheological fluids have flow characteristics of a convention oil.However, in the presence of a magnetic field, the particles becomepolarized so as to be organized into chains of particles within thefluid. The chains of particles act to increase the fluid shear strengthor flow resistance of the fluid. When the magnetic field is removed, theparticles return to an unorganized state and the fluid shear strength orflow resistance of the fluid returns to its previous value. Thus, thecontrolled application of a magnetic field allows the fluid shearstrength or flow resistance of a magnetorheological fluid to be alteredvery rapidly. Magnetorheological fluids are described in U.S. Pat. No.5,382,373 (Carlson et al.), hereby incorporated by reference in itsentirety. Suitable magnetorheological for use in the current inventionare commercially available from Lord Corporation of Cary, N.C.

[0035] A central passage 42 is formed in the housing 14 through whichthe drilling mud 5 flows. A pump 40, which may be of the Moineau type,and a directional electronics module 30 are supported within the passage42. As shown best in FIGS. 4 and 6, the pump 40 has an outlet 54 thatdirects the magnetorheological fluid outward through a radiallyextending passage 74 formed in the housing 14. From the passage 74, themagnetorheological fluid enters a supply manifold 62′ formed in thechamber 29′ that is axially aligned with the bank of pistons 12′. Twoother supply manifolds 62″ and 62′″ are formed within the chambers 29″and 29′″ so as to be axially aligned with the other two banks of pistons12″ and 12′″, respectively. From the supply manifold 62′, themagnetorheological fluid is divided into three streams.

[0036] As shown in FIG. 4, the first stream flows through opening 66′into tubing 51′ and then to a first supply valve 70′. As shown in FIGS.4 and 8, the second stream flows through a circumferentially extendingsupply passage 78 formed in the housing 14 to the second supply manifold62″. As shown in FIGS. 4 and 6a, from the supply manifold 62″ the secondstream of magnetorheological fluid flows through opening 66 into tubing51″ and then to a second supply valve 70″. Similarly, the third streamflows through circumferentially extending supply passage 80 to the thirdsupply manifold 62′″, then through opening 66′″ into tubing 51′″ andthen to a third supply valve 70′″. The supply valves 70 are discussedmore fully below.

[0037] As shown in FIGS. 4 and 6a , sections of tubing 53 are connectedto each of the three supply valves 70 and serve to direct themagnetorheological fluid from the supply valves to three axiallyextending supply passages 22 formed in the housing 14. Each supplypassage 22 extends axially underneath one bank of pistons 12 and thenturns 180° to form a return passage 24, as shown best in FIG. 10. Asshown in FIGS. 3 and 4, radial passages 23 direct the magnetorheologicalfluid from the each of the supply passages 22 to the cylinders 19 inwhich the pistons 12 associated with the respective bank of pistonsslide.

[0038] As shown in FIGS. 4 and 6a , the return passage 24 for each bankof pistons 12 delivers the magnetorheological fluid to a section oftubing 57 disposed within the chamber 29 associated with that bank ofpistons. The tubing 57 directs the fluid to three return valves 71, onefor each bank of pistons 12. From the return valves 71, sections oftubing 55 direct the fluid to openings 68 and into three returnmanifolds 64. As shown in FIG. 9, passages 79 and 83 direct the fluidfrom the return manifolds 64′ and 64′″ to the return manifold 64″ sothat return manifold 64″ receives the fluid from all three piston banks.As shown in FIG. 7, from the return manifold 64″, the fluid is directedby passage 76 to the inlet 56 for the pump 40 where it is recirculatedto the pistons 12 in a closed loop.

[0039] In operation, the pressure of the rheological fluid supplied tothe cylinders 19 for each bank of pistons 12 determines the magnitude ofthe radially outward force that the pistons in that bank exert againstthe springs 20 that bias them radially inward. Thus, the greater thepressure supplied to the pistons 12, the further the pistons extend andthe greater the radially outward force F that they apply to the walls ofthe bore 4. As discussed below, the pressure supplied to the pistons iscontrolled by the supply and return valves 70 and 71, respectively.

[0040] A supply valve 70 is shown in FIGS. 11 and 12. The valve 70 iselectromagnetically operated and preferably has no moving parts. Thevalve 70 comprises an inlet 93 to which the supply tubing 51, which isnon-magnetic, is attached. From the inlet 93, the Theological fluidflows over a non-magnetic end cap 89 enclosed by an expanded portion 86of tubing 57. From the end cap 89, the rheological fluid flows into anannular passage 94 formed between a cylindrical valve housing 87, madefrom a magnetic material, and a cylindrical core 92. The core 92 iscomprised of windings 99, such as copper wire, wrapped around a corebody 91 that is made from a magnetic material so as to form anelectromagnet. From the annular passage 94, the rheological fluid flowsover a second end cap 90 enclosed within an expanded section of thetubing 53, both of which are made from a non-magnetic material, and isdischarged from the valve 20. Preferably, the magnetic material in thevalve 70 is iron. A variety of materials may be used for thenon-magnetic material, such as non-magnetic stainless steel, brass,aluminum or plastic. The return valves 71, which in some applicationsmay be dispensed with, are constructed in a similar manner as the supplyvalves 70.

[0041] When electrical current flows through the windings 99, a magneticfield is developed around the core 92 that crosses the flow path in thepassage 94 in two places at right angles. The strength of this magneticfield is dependent upon the amperage of the current supplied to thewindings 99. As previously discussed, the shear strength, and thereforethe flow resistance, of the magnetorheological fluid is dependent uponthe strength of the magnetic field—the stronger the field, the greaterthe shear strength.

[0042]FIGS. 14 and 15 show an alternate embodiment of the supply andreturn valves 70 and 71. In this embodiment, the valve body consists ofa rectangular channel 104 made from a magnetic material and havingnon-magnetic transition sections 106 and 108 at its inlet and outletthat mate with the tubing sections 51, 53, 55 and 57. The channel 104 isdisposed within an electro-magnet formed by a C-shaped section ofmagnetic material 102 around which copper windings 110 are formed.

[0043]FIG. 16 shows the portion of the drill string 6 in the vicinity ofthe guidance module 10. In addition to the pump 40 and directionalelectronics module 30, previously discussed, the guidance module 10 alsoincludes a motor 116, which is driven by the flow of the drilling mudand which drives the pump 40, a bearing assembly 114, and an alternator112 that provides electrical current for the module.

[0044] According to the current invention, actuation of the pistons 12is controlled by adjusting a magnetic field within the valves 70 and 71.Specifically, the magnetic field is created by directing electricalcurrent to flow through the windings 99. As previously discussed, thismagnetic field increases the shear strength, and therefore the flowresistance, of the rheological fluid.

[0045] As shown in FIGS. 11 and 13, the flow of electrical current tothe windings 99 in each of the valves 70 and 71 is controlled by acontroller 13, which preferably comprises a programmable microprocessor,solid state relays, and devices for regulating the amperage of theelectrical current. Preferably, the controller 30 is located within thedirectional electronics module 30, although it could also be mounted inother locations, such as an MWD tool discussed below.

[0046] As shown in FIG. 4, the directional electronics module 30 mayinclude a magnetometer 123 and an accelerometer 124 that, usingtechniques well known in the art, allow the determination of the angularorientation of a fixed reference point A on the circumference of thedrill string 6 with respect to the circumference of the bore hole 4,typically north in a vertical well or the high side of the bore in ainclined well, typically referred to as “tool face”. For example, asshown in FIG. 2, the reference point A on the drill string is located atthe 0° location on the bore hole 4. The tool face information istransmitted to the controller 13 and allows it to determine theinstantaneous angular orientation of each of the piston banks—that is,the first bank of pistons 12′ is located at the 90° location on the borehole 4, etc.

[0047] Preferably, the drill string 6 also includes an MWD tool 118,shown in FIG. 16. Preferably, the MWD tool 118 includes an accelerometer120 to measure inclination and a magnetometer 121 to measure azimuth,thereby providing information on the direction in which the drill stringis oriented. However, these components could also be incorporated intothe directional electronics module 30. The MWD tool 118 also includes amud pulser 122 that uses techniques well known in the art to sendpressure pulses from the bottom hole assembly to the surface via thedrilling mud that are representative of the drilling direction sensed bythe directional sensors. As is also conventional, a strain gage basedpressure transducer at the surface (not shown) senses the pressurepulses and transmits electrical signals to a data acquisition andanalysis system portion of the surface control system 12 where the dataencoded into the mud pulses is decoded and analyzed. Based on thisinformation, as well as information about the formation 2 and the lengthof drill string 6 that has been extended into the bore 4, the drillingoperator then determines whether the direction at which the drilling isproceeding should be altered and, if so, by what amount.

[0048] Preferably, the MWD tool 118 also includes a pressure pulsationsensor 97 that senses pressure pulsations in the drilling mud flowing inthe annular passage 30 between the bore 4 and the drill string 6. Asuitable pressure pulsation sensor is disclosed in U.S. patentapplication Ser. No. 09/086,418, filed May 29, 1999, entitled “MethodAnd Apparatus For Communicating With Devices Downhole in a WellEspecially Adapted For Use as a Bottom Hole Mud Flow Sensor,” herebyincorporated by reference in its entirety. Based on input from thedrilling operator, the surface control system 12 sends pressure pulses126, indicated schematically in FIG. 13, downhole through the drillingmud 5 using a pressure pulsation device 132, shown in FIG. 1. Thepulsations 126 are sensed by the pressure sensor 97 and containinformation concerning the direction in which the drilling shouldproceed. The information from the pressure sensor 97 is directed to theguidance module controller 13, which decodes the pulses and determines,in conjunction with the signals from the orientation sensors 120 and 121and the tool face sensors 123 and 124, the sequence in which the pistons12 should be extended and, optionally, the amount of the change in thepressure of the rheological fluid supplied to the pistons 12.

[0049] The controller 13 then determines and sets the current suppliedto the supply and return valves 70 and 71, respectively, thereby settingthe strength of the magnetic field applied to the rheological fluid,which, in turn, regulates the pressure of the rheological fluid and theforce that is applied to the pistons 12. For example, with reference toFIG. 2, if the surface control system 12 determined that the drillingangle should be adjusted toward the 270° direction on the bore hole 4and transmitted such information to the controller 13, using mud flowtelemetry as discussed above, the controller 13 would determine that thepistons in each piston bank should be extended when such pistons reachedthe 90° location.

[0050] According to the current invention, the force exerted by thepistons 12 is dependent upon the pressure of the rheological fluid inthe piston cylinders 19, the greater the pressure, the greater the forceurging the pistons radially outward. This pressure is regulated by thesupply and return valves 70 and 71.

[0051] If it is desired to decrease the rheological fluid pressure inthe cylinders 19 associated with a given bank of pistons 12, current isapplied (or additional current is applied) to the windings of the valve70 that supplies rheological fluid to that bank of pistons so as tocreate (or increase) the magnetic field to which the rheological fluidis subjected as it flows through the valve. As previously discussed,this magnetic field increases the fluid shear strength and flowresistance of the rheological fluid, thereby increasing the pressuredrop across the valve 70 and reducing the pressure downstream of thevalve, thereby reducing the pressure of the rheological fluid in thecylinders 19 supplied by that valve. In addition, the current to thewindings in the return valve 71 associated with that bank of pistons isreduced, thereby decreasing the fluid shear strength and flow resistanceof the return valve 71, which also aids in reducing pressure in thecylinders 19.

[0052] Correspondingly, if it is desired to increase the rheologicalfluid pressure in the cylinders 19 associated with a given bank ofpistons 12, current is reduced (or cut off entirely) to the windings ofthe valve 70 that supplies rheological fluid to that bank of pistons soas to reduce (or eliminate) the magnetic field to which the rheologicalfluid is subjected as it flows through the valve. As previouslydiscussed, this reduction in magnetic field decreases the fluid shearstrength and flow resistance of the rheological fluid, therebydecreasing the pressure drop across the valve 70 and increasing thepressure downstream of the valve, thereby increasing the pressure of therheological fluid in the cylinders 19 supplied by that valve. Inaddition, the current to the windings in the return valve 71 associatedwith that bank of pistons is increased, thereby increasing the fluidshear strength and flow resistance of the return valve 71, which alsoaids in increasing pressure in the cylinders 19. Since the pressuregenerated by the pump 40 may vary, for example, depending on the flowrate of the drilling mud, optionally, a pressure sensor 125 isincorporated to measure the pressure of the rheological fluid suppliedby the pump and this information is supplied to the controller 13 so itcan be taken into account in determining the amperage of the current tobe supplied to the electromagnetic valves 70 and 71. In addition, theabsolute pressure of the magnetorheological fluid necessary to actuatethe pistons 12 will increase as the hole get deeper because the staticpressure of the drilling mud in the annular passage 130 between the bore4 and the drill string 6 increases as the hole get deeper and the columnof drilling mud get higher. Therefore, a pressure compensation systemcan be incorporated into the flow path for the magnetorheological fluidto ensure that the pressure provided by the pump is additive to thepressure of the drilling mud surrounding the guidance module 10.

[0053] Thus, by regulating the current supplied to the windings of thesupply and return valves 70 and 71, respectively, the controller 13 canextend and retract the pistons 12 and vary the force F applied by thepistons to the wall of the bore 4. Thus, the direction of the drillingcan be controlled. Moreover, by regulating the current, the rate atwhich the drill bit changes direction (i.e., the sharpness of the turn),sometimes referred to as the “build rate,” can also be controlled.

[0054] In some configurations, the drilling operator at the surfaceprovides instructions, via mud flow telemetry as discussed above, to thecontroller 13 as to the amount of change in the electrical current to besupplied to the electromagnetic valves 70 and 71. However, in analternative configuration, the drilling operator provides the directionin which the drilling should proceed. Using a feed back loop and thesignal from the directional sensors 120 and 121, the controller 13 thenvaries the current as necessary until the desired direction is achieved.

[0055] Alternatively, the drilling operator could provide instructions,via mud flow telemetry, concerning the location to which the drillshould proceed, as well as information concerning the length of drillstring that has been extended into the bore 4 thus far. This informationis then combined with information from the direction sensors 120 and 121by the controller 13, which then determines the direction in which thedrilling should proceed and the directional change necessary to attainthat direction in order to reach the instructed location.

[0056] In all of the embodiments described above the transmission ofinformation from the surface to the bottom hole assembly can beaccomplished using the apparatus and methods disclosed in theaforementioned U.S. patent application Ser. No. 09/086,418, filed May29, 1999, entitled “Method And Apparatus For Communicating With DevicesDownhole in a Well Especially Adapted For Use as a Bottom Hole Mud FlowSensor,” previously incorporated by reference in its entirety.

[0057] In another alternative, the controller 13 can be preprogrammed tocreate fixed drilling direction that is not altered during drilling.

[0058] Although the use of a magnetorheological fluid is preferred, theinvention could also be practiced using electrorheological fluid. Insuch fluids the shear strength can be varied by using a valve to applyan electrical current through the fluid.

[0059] Although the invention has been described with reference to adrill string drilling a well, the invention is applicable to othersituations in which it is desired to control the direction of travel ofa device through a passage, such as the control of drilling completionand production devices. Accordingly, the present invention may beembodied in other specific forms without departing from the spirit oressential attributes thereof and, accordingly, reference should be madeto the appended claims, rather than to the foregoing specification, asindicating the scope of the invention.

What is claimed:
 1. A guidance apparatus for steering a rotatable drillstring through a bore hole, comprising: a) a housing for incorporationinto said drill string; b) a movable member mounted in said housing soas to be capable of extending and retracting in the radial direction,said movable member having a distal end projecting from said housingadapted to engage the walls of said bore hole; c) a supply of amagnetorheological fluid; d) means for pressurizing saidmagnetorheological fluid; e) means for supply said pressurizedrheological fluid to said movable member, the pressure of saidrheological fluid generating a force urging said movable member toextend radially outward, the magnitude of said force being proportionalto the pressure of said rheological fluid supplied to said movablemember; and f) a valve for regulating the pressure of saidmagnetorheological fluid supplied to said movable member so as to altersaid force urging said movable member radially outward, said valvecomprising means for subjecting said magnetorheological fluid to amagnetic field so as to change the shear strength thereof.
 2. Theguidance apparatus according to claim 1, wherein said movable member isa piston slidably mounted in said housing.
 3. The guidance apparatusaccording to claim 1, wherein said pressurized fluid supply meanscomprises a passage placing said pressurizing means in fluid flowcommunication with said movable member, and wherein said valve isdisposed in said passage.
 4. The guidance apparatus according to claim1, further comprising: g) a second movable member mounted in saidhousing so as to be capable of extending and retracting in the radialdirection, said second movable member having a distal end projectingfrom said housing that is adapted to engage the walls of said bore hole,said second movable member being circumferentially spaced from saidfirst movable member; h) means for supply said pressurized rheologicalfluid to said second movable member; and i) a second valve forregulating the pressure of said magnetorheological fluid supplied tosaid second movable member so as to alter said force urging said secondmovable member radially outward, said second valve comprising means forsubjecting said magnetorheological fluid to a magnetic field so as tochange the shear strength thereof.